1. Field of the Invention
The present invention may be in the field of optical non-uniformity correction in a non-scanning, i.e. staring focal plane imaging system, but is not limited thereto.
2. Description of the Prior Art
Due, in part, to the successful development of charge coupled devices (CCD's), there is an increased interest in non-scanned, i.e. staring imaging systems. Although these systems benefit from decreased mechanical complexity, they encounter significant problems when imaging low contrast scenes. Since many scenes of interest, such a far infrared scenes or visible scenes in the presence of fog or smoke, are of low contrast, some mechanism for suppressing the high background level is necessary. In a staring array, the simplest method of suppressing the background level is to subtract a fixed value in the output signal from each pixel. Unfortunately, this thresholding process requires extreme focal plane uniformity because it does not distinguish between small signals due to input flux differences and those due to non-uniformities in the focal plane itself such as, doping level or depth errors and crystalline faults. Further, in a CCD based staring array there is a second problem because the charge storage well is quickly filled by the high background flux. Although this problem can be alleviated by reading the array at a very high sampling rate, this sampling rate imposes a high speed requirement on any non-uniformity correcting mechanism, and thus causes a severe constraint on electronic correction systems which require high precision digital-to-analog converters.
The optical non-uniformity correction technique described herein below is independent of the sampling rate and is capable of correction to a much greater precision than possible with a simple responsivity mask.